The Park Slope library branch renovation is going to take longer — and cost more — than expected. While early reports said that the branch would be closed for no more than 18 months, library officials now say that the Park Slope spot, 431 Sixth Ave., will be closed for at least two years, if not longer. What was supposed to be a routine renovation to make the 103-year-old facility handicap accessible has transformed into an all-building overhaul, including the installation of new floors, lighting and furniture. The cost of the renovations has subsequently jumped to $2.7 million, from $2 million, the price officials originally stated.